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Despite recent experiments exhibiting an impressive enhancement in radiative heat flux between parallel planar silica surfaces with gap sizes of about 10 nm, the exploration of sub-nanometric gap distances remains unexplored. In this work, by employing non-equilibrium molecular dynamics (NEMD) simulations, we study the heat transfer between two SiO2 plates in both their amorphous and crystalline forms. When the gap size is 2 nm, we find that the heat transfer coefficient experiences a substantial ∼30-fold increase compared to the experimental value at the gap size of 10 nm confirming the dependence on the distance inversely quadratic as predicted by the fluctuational electrodynamics (FE) theory. Comparative analysis between NEMD and FE reveals a generally good agreement, particularly for amorphous silica. Spectral heat transfer analysis demonstrates the profound influence of gap size on heat transfer, with peaks corresponding to the resonances of dielectric function. Deviations from the fluctuational electrodynamics theory at smaller gap sizes are interpreted in the context of acoustic phonon tunneling and the effects of a gradient of permittivity close to the surfaces. [ABSTRACT FROM AUTHOR]

This study investigates the impact of arrangement of two cylinders on their flow-induced vibrations (FIV) and heat transfer behavior at a Reynolds number of 100. Both cylinders were allowed to vibrate in two degrees of freedom (2DOF), encompassing streamwise and transverse directions. The arrangement of identical circular cylinders was varied across tandem (α = 0°), staggered (α = 30°, 45°, 60°), and side-by-side (α = 90°) configurations, at a constant center-to-center distance of 6D. The cylinders were heated at a fixed temperature to observe the forced convection heat transfer behavior under the influence of 2DOF FIV. To observe the FIV, the reduced velocity was varied from Ur = 0 (stationary cylinders) to 14. Results unveiled cylinder response sensitivity, encompassing vibration and heat transfer, with respect to reduced velocities and arrangements. Tandem arrangement exhibited the greatest vibrations for both cylinders. While lower drag was experienced in tandem for cylinder 2 (C-2), it escalated in staggered positioning. Both cylinders experienced lock-in between Ur = 6 and 8 for all arrangements, involving significant transverse vibration amplitudes. Maximum streamwise vibration reached 6.07% of the maximum transverse vibration for C-2 and 2.34% for C-1. Distinct slender "figure-8" and "oval-shaped" cylinder trajectories emerged, accompanied by diverse vorticity patterns in cylinder wakes across arrangements. For α = 60°, C-2 experienced 75.3% lower transverse vibration and 9.4% higher average Nusselt number compared to tandem setup. Overall, a pronounced correlation emerged between cylinder hydrodynamic behavior and heat transfer characteristics, evident through cylinder vibration, vortex shedding, average Nusselt number, and temperature distribution results. [ABSTRACT FROM AUTHOR]

Concentric tube heat exchangers are vital in various industrial applications, including chemical, process, energy, mechanical, and aeronautical engineering. Advancements in heat transfer efficiency present a significant challenge in contemporary research and development. This study concerns optimizing flow and heat transfer in concentric tube heat exchangers by morphing the tube's walls. The adjoint shape optimization approach is implemented in a fully turbulent flow regime. The effect of inner tube deformation on flow physics and heat transfer is examined. The results show that morphing can lead to a 54% increase in the heat transfer rate and a 47% improvement in the overall heat transfer coefficient compared to straight concentric tube designs. Moreover, the thermal-hydraulic performance factor is calculated to account for the relative increase in heat transfer when the optimal and initial designs are operated under the same pumping power. A thermal-hydraulic performance factor of 1.2 is obtained for the new design, showing that the heat transfer enhancement caused by morphing the tube's walls outweighs the increase in pumping power. The physics of a radial flow, resulting from an adverse pressure gradient in an annular region caused by the successive inner tube deformation, significantly augments heat transfer. This study shows morphing can lead to higher thermal efficiencies, and numerical optimization can assist in achieving this goal. [ABSTRACT FROM AUTHOR]

The heat transfer and wake-induced vibrations of a cylinder of circular cross section in the wake of another identical cylinder are numerically studied in this work at a Reynolds number (Re) = 100. The reduced velocities (Ur) are varied in the range of 2–14. The downstream cylinder is allowed to oscillate in two degrees of freedom, i.e., in the transverse as well as in the streamwise direction. The mass ratio (m*) is taken as 10, while the structural damping is ignored to get the maximum amplitude of vibration. The spacing ratio (L/D) between the cylinders is varied from 1.5 to 6, covering the major regimes, i.e., single body, reattachment, and co-shedding. The coefficients of lift (CL) and drag (CD), vibrational amplitudes of the cylinder, the Nusselt number (Nu), the Strouhal number (St), and vortex shedding patterns are studied. The results are discussed with the help of lift-displacement phase plots, cylinder trajectory plots, and vorticity and temperature contours. The lock-in condition at Ur = 8 is observed for all values of L/D, whereas the lock-in zone is the widest for the co-shedding regime at L/D = 6. By increasing L/D from 1.5 to 2.5 at Ur = 8, the CL of the downstream cylinder increases by 43%, whereas the CL of the upstream cylinder decreases by 61%. The downstream cylinder experiences lower drag as compared to the upstream cylinder and stationary isolated cylinder. A maximum decrease in the average drag coefficient of 107%, as compared to the stationary isolated cylinder, was observed for the downstream cylinder at L/D = 1.5 and Ur = 2, leading to the negative drag. Mostly, the 2S and C(2S) vortex shedding pattern is observed, whereas a steady flow and chaotic pattern emerged in a few cases. The results reveal that with increasing L/D, the average Nu for both the upstream and downstream cylinders increases as the effect of each cylinder on the other diminishes. [ABSTRACT FROM AUTHOR]

Understanding the behavior of viscoelastic (VE) fluids in confined geometries is crucial for applications in biologic systems, heat transfer devices, enhanced oil recovery, and many others. Here, we perform a systematic steady-state simulation of a VE fluid at low Reynolds numbers through a channel with successive smooth contractions and expansions. We analyze the hydrodynamic performance of the fluid with particular attention to vortex patterns that develop downstream of the contractions. We show that elastic vortices form at higher contraction ratios and that there are critical Weissenberg numbers (Wic) unique to each contraction ratio where the flow shifts from non-vortical to vortical. This Wic increases with an increasing contraction length. The coexistence of elongational-, shear-, and rotational-flow is essential for vortex development and evolution. We also analyzed the effect of the Deborah number (De) on the vortex pattern in a multiple contraction system and observed that the vortex area significantly depends on the distance between the contractions. We show that there are three distinctly different regions in De, in which the flow characteristics change in successive contractions. For high De, the flow in the downstream contraction is significantly affected by the upstream contraction. Our results have implications for the use of VE fluids with various VE properties in different types of porous media. [ABSTRACT FROM AUTHOR]

Present communication explores a novel application of the computational intelligence technique, namely, the Levenberg–Marquardt scheme under a Backpropagated Neural Network (LM-BNN) to solve the mathematical model for the magnetohydrodynamic peristaltic transport of Reiner–Philippoff (R–Ph) pseudoplastic fluid considering the influences of Ohmic heating, mixed convection, and viscous dissipation through a symmetric channel. The R–Ph fluid model is used in this investigation to elucidate the non-Newtonian behavior of the fluid under consideration. The Reiner–Philippoff fluid model delineates the intricate relationship between stress and deformation rate within the fluid. There are a few studies available on the peristaltic transport of the Reiner–Philippoff fluid that do not incorporate Joule heating, mixed convection, and magnetic field effects. Therefore, a novel mathematical model is developed to employ an artificial neural network technique with a different approach that has not been examined before. The governing equations of the problem are simplified using long wavelength and low Reynolds number approximations, and the resulting system is numerically solved using the BVP4c scheme in MATLAB based on the shooting algorithm. Furthermore, a dataset is constructed through the BVP4c technique for the proposed LM-BNN, considering eight scenarios of peristaltic motion of the Reiner–Philippoff fluid model by varying the Bingham number, the Brinkman number, the Grashof number, the R–Ph fluid parameter, and the Hartmann number. The numerical dataset is divided into 15 % for testing, 15 % for training, and 70 % for validation, which are utilized in LM-BNN to analyze the numerical solutions and Levenberg–Marquardt neural networks (LM-NNs) predicted results. The consistency and effectiveness of LM-BNN are validated through regression analysis, stresses at the wall, error histogram, correlation index, heat transfer, and mean squared error based fitness curves, which vary from 10 − 3 → 10 − 11 . Variations in several flow parameters affecting temperature and velocity profiles are explained physically through graphs. Additionally, an analysis of heat transfer and stresses at the wall, including absolute errors, is provided through tables. The outcomes reveal that the improving Grashof number and the Hartmann number tend to increase the temperature profile. Tabular results indicate that rates of heat transfer improve when assigning higher values to the Hartmann number, the Brinkman number, and the Grashof number, whereas stresses at the wall decrease for the Reiner–Philippoff fluid parameter and the Bingham number. The error analysis of numerical simulations is a valuable step in determining whether the data obtained are reliable and accurate. In terms of absolute error, the disagreement between numerical solutions and those predicted by LM-NNs is approximately 10 − 5 → 10 − 11 . It is clear from this error analysis that LM-NNs predicted results are consistent and reliable. [ABSTRACT FROM AUTHOR]

Graphene and several other two-dimensional (2D) carbon-based structures, including C3N, C3B, C2N, C3N4 (s-triazine), and C3N4 (tri-triazine), have attracted considerable attention due to their excellent thermal, mechanical, and electrical properties. In this study, the correlations between the interfacial thermal conductance (ITC) across these 2D nanostructures and the lattice-dynamical properties of the system, including (i) the phonon density of state (DOS) overlap between the sides of the interface and (ii) the percentage of different classes of vibrational modes for each heterostructure, are investigated. The results show that the percentage of localized interfacial modes is a strong descriptor for predicting ITC—even more effective than the widely accepted phonon DOS overlap. Moreover, the ITC between all combinations of these six carbon-based materials, its dependence on the length of the heterostructure, and the thermal rectification effect present in each interface structure is studied using non-equilibrium molecular dynamics simulations. The results show that the maximum and minimum ITC for infinite length systems belong to graphene/C3N and C2N/C3N4 (tri-triazine) heterostructures with values of 35.81 and 2.21 GW m−2 K−1, respectively. Comparing the thermal resistances at the interface and in the bulk of these heterostructures show that thermal transport in carbon-based 2D materials is influenced by the thermal resistance across their interfaces. These results not only contribute to our fundamental understanding of interfacial heat transfer, but can also serve as the basis for the design of nanoelectronic devices based on 2D materials, where the device level performance will indeed be influenced by interfacial phenomena. [ABSTRACT FROM AUTHOR]

The current investigation is based on the impact of the nanoparticle shape on the micropolar hybrid nanofluid flow in a vertical plate. Furthermore, the aim of this investigation is to optimize the skin friction as well as the Nusselt number using a statistical approach known as "Response Surface Methodology" (RSM). The micropolar hybrid nanofluid is considered due to its enhanced thermal properties likely the Hamilton–Crosser thermal conductivity, Gharesim model viscosity, etc. A suitable similarity rule is adopted for the transformation of the designed model into ordinary and then solved numerically utilizing the shooting-based Runge–Kutta fourth-order technique. The simulated results of diversified parameters are presented through graphs. Furthermore, RSM is employed to design and develop a mathematical model to get an optimized hear transfer rate along with the rate of shear stress. The required components are carefully selected, and the corresponding responses are recorded. The collected data is subsequently employed in constructing a response surface through regression analysis. This process allows for the determination of optimal conditions to enhance heat transfer, which is then confirmed through analysis of variance testing. However, the major outcomes of the study are; for the case of suction with increasing particle concentrations, the shear rate, hear transfer rate, and couple stress coefficients are enhanced significantly. Furthermore, the non-Newtonian parameter and the magnetic parameter also favor in enhancing the rate coefficients. [ABSTRACT FROM AUTHOR]

Flow and heat transfer problems in non-Newtonian fluids are considerably more complicated compared to Newtonian fluids due to their non-linear behavior and complex properties. This paper numerically studied the flow and heat transfer of a viscoelastic fluid around a hot rotating cylinder in a square enclosure. The Phan-Thien–Tanner (PTT) model, one of the most accurate models available to describe the behavior of viscoelastic fluids, was used to obtain the constitutive equations of the viscoelastic fluid, which were then solved by using the finite-volume method with the Rheoheatfoam solver of the OpenFOAM software. The log-conformation approach was employed to avoid the divergence problem at high Weissenberg numbers. Simulations were conducted to investigate the effects of the Weissenberg number, Brinkman number, Reynolds number, and the ratio of the polymeric viscosity to total viscosity on the behavior of fluid flow and heat transfer. The results showed that increasing the Weissenberg number led to a higher shear thinning effect, which decreased the fluid's effective viscosity and viscous dissipation near the cylinder. As a result, heat transfer from the cylinder increased, and the frictional torque applied to the cylinder decreased. However, the shear thinning effect was negligible when the Weissenberg number exceeded 100. Furthermore, the tangential normal stress significantly increased as the Weissenberg number increased up to one. However, with further increases in the Weissenberg number, the tangential normal stress reduced, eventually reaching close to zero at high Weissenberg numbers. [ABSTRACT FROM AUTHOR]

Device cooling schemes are required to reduce the local temperature of solar panels and solar air heaters, while maintaining their radiative heat to maximize energy conversion. Therefore, an efficient cooling scheme was developed using textured surfaces augmented by highly porous materials for increasing their total surface area. In this study, highly porous iron carboxylate framework, MIL-100(Fe), Materials of Institute Lavoisier, was introduced to substrates to provide a highly textured surface. This significantly reduced the temperature of the surface that was subject to radiative heat during both air and mist (or aerosol) cooling. In the case of mist cooling, the proposed MIL-100(Fe)-coated substrates were superhydrophilic, which promoted close contact between the impacting aerosols and the heated surface. Single drop impact and evaporation experiments were conducted to quantify the rate of heat removal provided by the proposed MIL-100(Fe) coatings. These coatings provided an increase in the Leidenfrost limit from 140 to 200 °C. As such, the highly wettable and porous MIL-100(Fe)-coated layers promoted rapid evaporative cooling. The proposed layers were analyzed using scanning electron microscopy, x-ray diffraction, atomic force microscopy, and Brunauer–Emmett–Teller data to elucidate the reason for their increased heat transfer rate. [ABSTRACT FROM AUTHOR]

In this work, an attempt has been made to explore numerically the thermo-fluidic transport process in a novel M-shaped enclosure filled with permeable material along with Al2O3-Cu hybrid nanoparticles suspended in water under the influence of a horizontal magnetizing field. To exercise the influence of geometric parameters, a classical trapezoidal cavity is modified with an inverted triangle at the top to construct an M-shaped cavity. The cavity is heated isothermally from the bottom and cooled from the top, whereas the inclined sidewalls are insulated. The role of geometric parameters on the thermal performance is scrutinized thoroughly by changing the sidewall inclination, number, and height of the top inverted triangular undulation under similar boundary conditions. The governing equations transformed into dimensionless form are solved by using a computing code written in the finite volume approach. The analysis is conducted by considering a wide range of parametric influences like sidewall angles (γ), number (n), and height (δ) of the top triangular undulations, modified Rayleigh number (Ram), Darcy number (Da), Hartmann number (Ha), and hybrid nanoparticle concentrations (φ). Furthermore, the artificial neural network (ANN) technique is implemented and tested to predict the overall thermal behavior of the novel cavity to predict new cases. The results revealed that the design of sidewall inclination (γ) is an important parameter for modulating the thermo-flow physics. The M-shaped cavity (compared to trapezoidal) reveals either a rise or drop in the fluid circulation strength depending upon the magnitude of δ, but the heat transfer rate always increases due to an increase in the cooling length. The heat transfer increment is ∼61.01% as δ increases. Single undulation with higher depth is the optimum choice for achieving improved heat transfer (which may go up to ∼355.75% for δ = 0.5 and γ = 45°). A decrease in Da or Ha causes a drop in the flow strength, which consequently leads to a drop in the heat transfer rate. Furthermore, the concepts of ANN will help researchers predict the behavior for such complicated cavity shapes with a multiphysics approach. This will save efforts as well as computing time for exploring the thermal behavior of any range of a dataset. [ABSTRACT FROM AUTHOR]